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Babingtonite and Epidote from Westfield, Massachusetts

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Babingtonite and Epidote from Westfield, Massachusetts BABINGTONITE AND EPIDOTE FROM WESTFIELD, MASSACHUSETTS Cnanlos Palacno, H antar iI U niver si.ty,C ambrid, ge, M ass achus et'ts In 1932 I read a paper on babingtonite before the Mineralogical Societyl describing several new occurrencesof this rare mineral. Con- cerning one of these, Holyoke, Mass., I stated "In abundance and beauty it far surpassesany previously described occurrence of babing- tonite." This statement is no longer true, at least as regards the beauty of specimens,since the babingtonite from Westfield, Mass., far surpasses that of Holyoke in attractiveness. A group of students of mineralogy from Harvard University, visiting the well-known trap quarry at Westfield in the spring of 1935, brought back a number of specimens which proved to contain babingtoni e, hitherto not reported from this locality.2 The specimensconsist of veins in the diabase, narrow for the most part but expanding to a width of three or four inches. The veins appear, from the hand specimens col- lected, to be of no great lateral extent and vary widely in paragenesisas revealed by the following partial list of mineral sequences. Quartz, calcite Calcite only Datolite only Calcite, datolite, apophyllite Qrartz, chlorite, calcite, prehnite Quattz, chlorite, epidote (iron-rich), prehnite Calcite, epidote (iron-poor), datolite Qraartz, chlorite, sphalerite, chalcopyrite, calcite Chlorite, calcite, babingtonite Calcite, babingtonite Quartz, prehnite, babingtonite, calcite Ring structures, epidote, datolite, epidote, prehnite, datolite It is clear from this list that the sequencegenerally begins with either qrartz or calcite, and calcite is the most abundant mineral, in many casescompletely filling the vein in coarse granular aggregates.All the minerals present except datolite being little attacked by acid, it was possible to remove the calcite with dilute hydrochloric acid, thus re- vealing the earlier minerals. The most striking specimen, shown in the photograph, is a vein sec- 1 Palache, C., and Gonyer, F. A., On babingtonite: Am. Mineral., vol. 17, p. 295, t932. 2 Shannon, Earl V., Famous mineral localities. The datolite locality near Westfield, Maps.; Arn. Mineral., vol. 4, p. 5,1919. 6s2 TOURNALMINERALOGICAL SOCIETY OF AMERICA 653 tion about nine inches square. After treatment with acid it showed a thin wall-coating of granular qttartz upon which rests prehnite of a rich green color, coarsely crystallized in rudely cone-shapedaggregates an inch across. Upon the prehnite the babingtonite stands in clusters of rough crystals, velvety black in color and measuring from a half to three- quarters of an inch in diameter. The color contrast produces a speci- men of most uncommonbeautv. Babingtonite and Prehnite from Westfield, Massachusetts The babingtonite crystals are curved and indistinct, but a study oI some of the smaller onesby Mr. W. E. Richmond, Jr. establishedthe presenceof the forms:-c(001), D(010),a(100), h(ll}), g(2I0), JG20), o(011),s(01 1), and d(101), all known from the Holyoke specimensand in similar development.s Analysis of a sample of very fresh crystals by F. A. Gonyer yielded the composition of column 1. 3 Loc. cit., p. 297 and Fig. 3. 654 TH E AMERICAN M I N ERALOGIST Alllt.rvsts oF BABrNcroNrrE l. 2. sio2 52.32 52.44 Tio2 0.15 abs. AlzOt 0.95 0.23 FezOa 13.42 74.43 FeO 10.65 10.12 MnO 0.31 1.03 Mgo 0.50 0.65 CaO 19.88 19.67 NazO 0.27 0.08 KrO abs. abs. HzO* 1.36 1.27 99J1 gg.f,, 1. Westfield, Mass., Analyst, F. A. Gonyer 2. Holyoke, Mass., Analyst, F. A. Gonyer Comparison of the two analyses shows how closely the two occur- rences conform in chemical nature. Once more we have strong evidence of the essentialconstancy in composition of this complex silicate.a The associated minerals except epidote present little that is note- worthy. The chlorite, sparsely present in a few specimens, is in the form of bronzy scales.It is probably the same chlorite as described by Shannonsunder the name chalcodite. Datolite is in part massive, in part in water-white, glassy crystals of great complexity of development. Epidote is in two generations,but as they do not occur together their relative age is unknown. The one is of an ordinary yellowish green color and shows no distinctive features. The other is deep black in color, simulating babingtonite closely. This epidote was analyzed by Mr. Gonyer and studied optically by Dr. Berman, to whom I am indebted for the data presented below. ANarvsts ol Eproorn tnoM Wnsrrrslo, Mess. SiOz 36.52 Tio: abs. ALOs 20.97 Fezo: r7.22 FeO 0.45 MnO abs. Mgo abs. CaO 23.05 HzO* 1.98 100.19 D.:3.49 (Berman) a Loc. cit , p. 299. 6 Shannon, E. V., Diabantite, stilpnomelane and chalcodite from Westfield: Prac. United. States N al. M us., 57, p. 397, 1920. JOURNAL MINERALOGICAL SOCIETY OF AMERICA 655 Discussionof the analysis yields the formula: Ca2(Al,Fe)gSiaOrr(OH) with Al/Fe : 1.91 The optical data obtained by Dr. Berman are;-a:I.75I, P:1.784, -l t : 1.797 ; all + .002.B $ (- ), 2V : 64" 2o,r ) a. X : colorless; Y : greenI Z: olive green. As this epidote is high in iron, an attempt was made to determine how it would modify the published curves relating iron content and re- fractive indices. It was found, however, that the data in this region of the curve were too few and too contradictory to permit the drawing of a curve with any real meaning. It is evident that more data obtained from analyzed material are necessarynot only in the iron-rich epidotes but throughout the series..
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